Used humorously, to go native means to take on some (or all) of the culture traits of the people around you, often said of people who go to foreign countries or far away cities. These traits may include dress, language, accent, etiquette, religion, etc.

Important: The common use of the phrase does not have anything to do Native Americans.

A stupid person; it refers to the lack of surface area on an individual's brain. The general thought is that the more surface area (wrinkles, creases, etc.) a brain has, the smarter the person is. Conversely, a person with a smooth brain (no wrinkles) has less surface area and would therefore be stupid.